Nonaqueous battery, especially lithium ion batteries, have characteristics of a small size, a light weight and a high energy density, and are used widely as a main power supply of small information-and-telecommunication instruments, such as a cellular phone and a personal computer, and also they are being adopted as a power supply of an electric vehicle or a hybrid vehicle from the viewpoints of resource and environmental problems.
The electrodes (a positive electrode and a negative electrode) of such a nonaqueous battery, are formed by applying an electrode mixture slurry obtained by mixing a binder (binding agent) with powdery electrode-forming materials, such as an electrode active substance and an optional electroconductive additive, dissolving or dispersing the mixture in an appropriate solvent to form an electrode mixture slurry, and applying the mixture slurry on an electroconductive substrate to form a composite electrode layer. The binder is required to show a durability against a nonaqueous electrolytic solution obtained by dissolving an electrolytes, such as LiPF6 or LiClO4, in a nonaqueous solvent, such as ethylene carbonate or propylene carbonate, and fluorine-containing polymers, such as tetrafluoroethylene polymer, styrene-butadiene copolymers, etc., have been used conventionally, whereas vinylidene fluoride polymers are being used widely in recent years, because of a smaller resistivity and good film-forming property.
However, in a nonaqueous battery, especially a lithium ion battery, the desire for a smaller size, a smaller weight and a higher energy density, is increasing in recent years, and some problems accompanying it are also recognized. Particularly, for realizing a smaller weight and a higher energy density, it becomes necessary to form a thick electrode mixture layer on an electroconductive substrate and to increase the proportion of the electrode active substance in the battery volume, whereas this is accompanied with an embrittlement of the electrode mixture layer, so that the electrode mixture layer is liable to collapse easily when subjected to deformation. Furthermore, in case where the electrode mixture layer is densified by pressing, the liability becomes more pronounced. The collapse of the mixture layer leads to short circuit between the electrodes and should be avoided strictly. Although these problems are not so serious with respect to a negative electrode mixture using a carbon or graphite-based active substance which has a certain degree of flexibility, but are liable to be pronounced about the positive electrode mixture which uses an active substance of lithium-based composite metal oxide which per se has little flexibility or lubricity and further includes an electroconductive additive.
Hitherto, several proposals have been made about such a positive electrode mixture. For example, there have been proposed a positive electrode mixture which comprises, in addition to a mixture of an active substance comprising a lithium nickel composite metal oxide, an electroconductive additive, a binder and a solvent, 0.1 to 3 wt. parts of an organic acid having a basicity of two or more per 100 wt. parts of the active substance (Patent document 1 below); and a positive electrode mixture for nonaqueous batteries which includes an organic acid in addition to a mixture of a positive electrode active substance of a composite metal oxide, an electroconductive additive, a vinylidene fluoride polymer, and an organic solvent (Patent document 2 below). However, even by using these positive electrode mixtures, there has not been realized a sufficiently thick positive electrode of a high energy density.
[Patent document 1] JP-A 2001-35495
[Patent document 2] JP-B 3540097